Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 7 - No. 9)
To solve for angles in a right-angled triangle, you can use:
Trigonometric Ratios
Inverse trigonometric functions
Knowledge of sides lengths
knowledge of the Pythagorean Theorem
Paskaidrojums
All the points can be used for solving angles in the triangle.